Here's how # 1 works:
Area is proportional to the square. Volume (and weight) are proportional to the cube. So, let's start with gradually increasing cubes, because cubes are the easiest.
Cube 1, has a side of 1". It's surface area is 6 sq in (6 sides, each 1 x 1), and it's volume is 1 cu in. Even though they're different units let's remember the ratio of 6 : 1. .
Cube 2 has a side of 2". It's surface area is 24 (6 x 2 x 2) and it's volume is 8. So the ration is 24 : 8 which is equal to 3 : 1.
Cube 3 has a side of 3". It's surface area is 54. It's volume is 27. So the ratio is 54 : 27 which simplifies to 2 : 1.
Cube 4, has a side of 4". It's surface area is 96. And its volume is 64. So the ratio of area to volume 96 : 64 or 3 : 2.
A square is like any other three dimensional solid, except that volumes and areas are a little easier to calculate. You can see that as area increases the ratio of area to volume actually decreases.
I've been avoiding going too deeply into #2 because it's not a simple explanation. It's easier to just say "biochemistry," and hope you take it on faith. But apparently that isn't going to happen.
There are a lot of processes which go on in cooking meat. Only one of them is really at issue, but it might be helpful to compare it to another.
When you cook a naturally tender piece of meat, you usually cook it long enough to kill any bacteria and convert some of the amino acids to sugars. As the meat cooks, it also contracts and firms up. This happens because protein molecules contract with heat. This is what happens when you cook meat rare to medium.
When you cook tough meats which have more than their share of a particular class of proteins called colloids, you can cook it "beyond done and into tender," by cooking it low and slow. After the proteins have been exposed to a certain minimum quanta of heat energy for a certain minimum amount of time, the protein strands actually start to relax and unwind. This is called denaturing. When this happens, the meat becomes tender.
For our purposes, the important thing to focus on is the requirement of a minimum amount of heat and time. It doesn't matter how small the piece of meat is. It has to be exposed to that minimum before the proteins will denature.
What this means as a practical matter is that a 3 lb brisket will take 4.5 hours to cook at 250 (because it takes a minimum of 4.5 hours to get brisket tender, or 1.5 hours/lb , and a 10 lb brisket will take 12.5 hours, or 1.25 lbs per pound. (These times are actually very close). Pork times are similar. You don't have to take these times on faith. If you get recipes from folks who cook a lot of pieces, like
www.virtualweberbullet.com you'll see that they allow a little more time per pound than people who cook full cuts.
Clearer now?
Rich
PS As an interesting aside: Once you get into really big cuts, like whole hogs or beef primals the time gets cut (proportionally) even shorter because the thickness of the meat is more important than its weight.